This invention relates generally to an apparatus and method for grinding a workpiece, and more particularly to the grinding of the base portion of turbine and compressor disks or rotors (which are hereafter collectively referred to as “turbine disks”) and related types of workpieces.
The base or root portions of turbine blades are generally formed with a series of shoulders which are closely fitted into profiles of the corresponding support portions of the turbine disks to make sure that the turbine blades are maintained securely in place during rotation of the disks.
Heretofore, the desired configuration of the profiles in the turbine disk have typically been formed using conventional broaching machines which form the profiles without cracking the parent material. Broaching machines, in general, include a cutter, and it consists of cutting teeth arranged in a row. The broaching machine works on the principal of proper offsetting of the workpiece and then performing work on it.
As a result, broaching machines are very expensive, very heavy so as to require special foundations, and involve a significant amount of time to reset the cutting members for each new job. Also, the cutting teeth require frequent resharpening.
When the profiles in a turbine disk are formed using a broaching machine, it is usually done in three sequential steps which are illustrated in FIG. 1 of this application. The first cut is the roughing broach, after which there is a semi-finishing broach, and a finishing broach, all as illustrated in FIG. 1, which is intended to be a diagrammatic view of a typical cutting sequence for a broaching machine. In some operations, the roughing broach is not carried out as a separate step.
In general, although the broaching process for forming the profiles for the profiles in a turbine disk is relatively fast and efficient, it has a number of disadvantages. First, the purchase price, maintenance cost, floor space requirements and long lead-time to obtain broaching machines is a significant disadvantage. Additionally, the machines must be supported on a special concrete base with other infrastructure to support broaching machines, which adds to the overall cost of the machines. Finally, the cutting elements of the broaching machine must be resharpened frequently, and a significant amount of time is required to set up and change over broaching machines to cut profiles with different profiles.
It is also known, generally, that profiles may be formed in metals and alloys using conventional grinding wheels, such as, for example, a grinding wheel having a fused aluminum oxide grinding surface. The grinding wheel is applied to the workpiece in a direction perpendicular to its axis of rotation and perpendicular or at a specified angle to the surface of the workpiece to be ground. As the grinding wheel is applied to the workpiece, the workpiece is ground away by the abrasive surface of the grinding wheel, which also generally results in very small loose particles or grits of the grinding wheel surface being separated from the grinding wheel. The grinding action of the grinding wheel may generate sufficient heat to actually “burn” the workpiece to the point at which the microstructure and properties of the workpiece are altered that can create small cracks or other undesirable thermal damage in the workpiece. As used herein the terms “burn” and “burning” shall mean the point at which the thermal effects of the grinding operation create detrimental thermal damage or adverse conversion of the material property of the workpiece, such as, for example, cracks. The adverse affects of the cracks and other thermal damage being formed as a result of excess heat generation are known, and they are exacerbated when the workpiece is a turbine disk for aircraft which, in use, is exposed to a wide range of heating and cooling during take-offs and landings, and these temperature variations and mechanical loads can cause small cracks to become larger and larger to a point where the safety of the aircraft could be affected. In a effort to avoid or reduce the burning of the workpiece, it is common practice to recognize the point at which burning may begin to occur, and reverse the movement of the grinding wheel in a direction away from the workpiece and, in some cases, to increase the amount of cooling liquid applied to the workpiece at the point where the grinding wheel is being applied. Additionally, because of the grinding action applied to the workpiece by a conventional grinding wheel, the grinding wheel must make a large number of passes through the workpiece, and the passes are carried out at reduced material removal rates so as to reduce the risks of excess heat generation, all of which results in a grinding operation that is very slow, and there is still a significant risk of creating cracks and thermal damage in the workpiece.
Finally, grinding wheels that have an extruded SG ALOx crystal of known aspect ratio and a ceramic bond material with an open structure are known, but heretofore they have had only limited applications, such as turbine blade root forms and other roughing operations.
Accordingly, a need exists for properly forming profiles in a turbine disk or related workpiece using a method and apparatus that avoids many of the disadvantages of the currently used broaching machines and conventional grinding wheels, and particularly for forming the profiles without cracking the parent material, or inducing other thermal damage thereto.